Influence of environmental relative humidity on the tensile and rotational behaviour of hemp fibres
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The aim of this study is to throw new light on the influence of moisture on the mechanical properties of hemp fibres. Indeed, the behaviour of plant-based fibres strongly depends on their humidity. Although this topic has been relatively well treated for the case of wood, the literature on fibre stemming from annual plants is unfortunately poor. This purpose is, however, of great importance, particularly in view of the production of high-performance composites. The influence of environmental conditions on the static and dynamic tensile moduli and the strength of elementary fibres are investigated using a versatile experimental setup. Novel equipment was also designed to measure the rotation of a fibre about its axis when it was subjected to static loading and moisture variations. Water sorption is shown to have a significant influence on the apparent tensile stiffness, strength and fracture mode of such fibres, and is also shown to act like an activator of the stiffening phenomena under cyclic loading. A remarkable increase in the fibre stiffness of up to 250% is measured. Significant longitudinal elongation, reaching a value in excess of 2%, is associated with this increase in stiffness. The absorption and desorption of moisture also lead to substantial rotation of the fibre about its axis. Water sorption certainly involves a modification of the adhesion between cellulose microfibrils and the amorphous matrix. Under cyclic loading, the cellulose microfibrils could be able to creep into the relaxed amorphous matrix, leading to their re-arrangement, with more parallel orientations with respect to the fibre axis.
KeywordsHemp fibres Tensile testing Water sorption Stiffening Damage DMA
The authors would like to thank Jean-Marc Côte and Camille Garcin from the FEMTO-ST for their assistance with some of the experiments, and Patrick Perré from the Ecole Centrale de Paris (Laboratoire de Génie des Procédés et Matériaux—Material Processes Engineering Laboratory) for their very fruitful and helpful discussions. We also thank Christine Millot for her technical contribution to the SEM characterisation of elementary fibres.
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